1. Field of the Invention
The present invention relates to a laser beam harmonics generator and a light exposing device.
2. Related Background Art
An ultra-high pressure mercury lamp has been primarily used as a light source of a light exposing device which is used in manufacturing a semiconductor device.
However, recently the semiconductor device is more and more highly integrated and finely fabricated and an optical exposing system has been expanding its application area more and more as a high resolution power lens was developed. In this type of light exposing device, when a mask or reticle circuit pattern is to be transferred and formed on a wafer, a resolution line width of the circuit pattern formed on the wafer is proportional to a wavelength of an exposing light. Accordingly, recently a KrF excimer laser is used in place of the ultra-high pressure mercury lamp. The utilization of a harmonic light produced by radiating a pulse laser, such as a metal vapor laser, onto a material which generates a harmonic, such as a non-linear optical crystal, has been studied.
However, it has been found that a light exposing device which uses the excimer laser has many problems. For example, in a stable resonator type excimer laser, spectrum width is as wide as .DELTA..lambda.=0.4 nm (that is, a monochrominance property is poor). Thus, where a monochromatic lens consisting of silica is used as a projecting optical system, it is necessary to narrow the wavelength. Common problems with the excimer laser, include short durability and difficulty of maintenance due to the use of noxious gas such as fluorine.
In order to avoid the above problems, the harmonics of the pulse laser have been studied. In order to generate a second harmonic wave, a single crystal of a dielectric material without inversion symmetry (such as non-linear optical crystal) is used. In order to generate the second harmonic wave from the single crystal of the dielectric material without the inversion symmetry, it is necessary to meet a phase matching condition 1 in which refractive indices of the crystal for a fundamental wave and the second harmonic wave are equal. To this end, the laser beam must be applied at a specific angle. If the phase matching condition is not met, efficiency of generation of the harmonics is lowered. Further, in order to improve the efficiency of generation of the second harmonic wave, it is necessary to increase energy density per unit area of the laser beam. However, the more the laser beam is focused in the crystal, the more does the crystal absorb the laser beam and a temperature distribution (temperature gradient) in the crystal becomes abrupt because of the heat generated by the absorption. Since the refractive index of the crystal is changed by the change of temperature, the phase matching condition is no longer met. Further, the higher the power applied to the laser beam per unit area of the crystal and per unit time, the shorter the durability of the crystal. In addition, since the second harmonic wave generated from the laser beam has a high coherency, an undesired interference pattern such as speckle or interference fringe occurs.